Retro-reflecting sheet material and method of making same

ABSTRACT

The specification relates to retro-reflectors of the type used for road signs and markers and vehicle license plates to make these objects more noticeable to road users after dark. In the invention, the surface of a metal substrate is made retro-reflective by applying a layer of an organic polymeric material and adhering glass beads thereto, and then passing the substrate through a roller mill or the like with the glass beads covered by a platen so that the beads form indentations in the metal surface. The presence of the platen allows the pressing (indentation) step to take place without damage to the glass beads or to the layer between the beads and the metal surface. By this means, a hard metal article can be made directly retro-reflective.

BACKGROUND OF THE INVENTION

This invention relates to retro-reflective sheet material (also known asreflex reflectors) and to a method of preparing the same.

Retro-reflective material is well known and widely used in the priorart, mainly in connection with road signs and automobile licence platesand the like. The material is capable of reflecting an incident beam oflight back in the general direction of the light source. When, forexample, a car headlight illuminates a road sign bearing such materialin otherwise dark surroundings, the sign reflects a greater amount oflight towards the driver than surrounding objects and is thereforeclearly visible.

Known retro-reflective materials fall into two general types. The firsttype consists of spherical glass beads adhered to the surface of atransparent organic coating applied over a reflective metal (or other)substrate but only partially imbedded in it so that a glass-airinterface is presented to incoming light. This type of material does notfunction as a retro-reflector when the surface is wet with water.

The second type of material consists of high refractive index (about 1.9and greater) glass beads suspended within a relatively thick film of atransparent organic coating (plastic film) applied over a reflectivemetal substrate. Such material retains its retro-reflective propertieswhen wet.

Theoretical considerations dictate that the most effectiveretro-reflectors of the second type can be formed by positioning theglass beads at just the right distance from conforming segments ofconcave spherical mirrors of corresponding size located behind theindividual beads. This optimum distance will vary depending on thediameter of the beads, their refractive index, and the refractive indexof the medium in which they are suspended.

Various known retro-reflectors embody such a structure but havegenerally been produced in the form of flexible tapes or sheets whichare adhered to the desired object, such as a road sign. The step ofadhering the tape to the object can be time consuming and uneconomical,and the tape may peel from the object after a period of exposure to theelements.

One example of known retro-reflectors is disclosed in U.S. Pat. No.2,407,680 issued on Sept. 17, 1946 and assigned to Minnesota Mining andManufacturing Company. This patent was one of the first to disclose thesecond type of structure referred to above employing high refractiveindex spheres, and it is to be noted that it suggests the use of apolished metal surface as the back reflector with the beads spaced anoptimum distance therefrom. The patent also suggests the formation ofconcave mirrors in a reflective surface formed by a reflective binderlayer for the heads.

U.S. Pat. No. 2,543,800 issued on Mar. 6, 1951 and assigned to MinnesotaMining and Manufacturing Company discloses a retro-reflector in whichthe beads are spaced a small distance from corresponding reflectorsurfaces formed by pressing the beads partially into a moldable cushionlayer having a reflective surface coating containing metallic flakepigment particles, the beads being spaced from the cushion layer by athin film which may contain a transparent pigment. After the pressingoperation the plastic layers are cured. One disadvantage of suchretro-reflectors is that the reflector surfaces formed by the cushionlayer are not as reflective as a polished metal surface and thereforelight is lost at these surfaces.

U.S. Pat. No. 3,922,433 issued on Nov. 25, 1975 and assigned to AluminumCompany of America relates to partially embedding the spherical glassbeads into a metallic coating while it is in the molten condition. Thisinvention is an attempt to form a retro-reflective surface directly on asubstrate made of a hard material, such as a road sign, without firstforming a flexible tape to be adhered thereto. An iron-base substrate isdipped into a molten bath of aluminum, zinc, tin, lead or alloys thereofand is sprayed with the glass beads by an air gun as the substrate iswithdrawn from the bath and the coating is still molten. This method hasthe disadvantages that it is expensive and the beads are not spaced fromthe reflective surface as is required for the optimum retro-reflection.

There is therefore a need for a method of producing a retro-reflectivesurface directly onto hard metal substrates, which method permits thebeads to be spaced by the optimum distance from conforming concavereflective surfaces.

The formation of a plastic layer containing the glass beads and thepressing of the beads into the metal substrate has been contemplatedbut, because of the relative hardness of the substrate surface, it hasbeen found that the beads tend to shatter and the layer of plasticbetween the beads and the substrate surface tends to become attenuatedor damaged when the plastic layer is rolled with sufficient force tocause the beads to indent the substrate surface. Moreover, the beadstend to become mis-aligned with the indentations in the surface so thata useless product is produced.

SUMMARY OF THE INVENTION

It has now unexpectedly been found that a product of high quality can beproduced by indenting the substrate surface by applying pressure to theglass beads when a platen is located between the surface of the layercontaining the glass beads and a roller or the like used for applyingsaid pressure.

Thus, according to one aspect of the invention, there is provided amethod of forming a retro-reflective surface on an indentable metalsubstrate, comprising the steps of: (a) forming on said indentable metalsubstrate a layer of transparent organic polymeric material having amono-layer coating of substantially spherical glass beads of highrefractive index; (b) covering said beads with a platen havingsubstantially no tendency to adhere to the glass beads under thepressures encountered; (c) applying sufficient pressure to said platenand indentable metal substrate to cause the glass beads to indent thesurface of said indentable metal substrate; and (d) covering said beadedlayer with a further layer of transparent organic polymeric material;said transparent organic polymeric material formed on the indentablemetal substrate in step (a) being suitable to withstand the pressure ofstep (c) without substantial crazing, cracking or attenuation, and thethickness of said layer being sufficient to space said beads from saidsubstrate by a predetermined distance suitable for retro-reflectionafter step (c).

According to another aspect of the invention there is provided aretro-reflector comprising an indentable metal substrate, a layer oftransparent organic polymeric material overlying a surface of thesubstrate, a mono-layer of glass beads of high refractive indexseparated from said substrate by said layer of transparent organicpolymeric material, and a further layer of transparent organic polymericmaterial overlying said mono-layer of glass beads, the surface of saidsubstrate having indentations conforming to the adjacent glass beads,and the separation of the glass beads from the conforming indentationsbeing sufficient for retro-reflection.

When a platen is used in the indentation step, it is found that, despitethe large pressure or force required to force the glass beads to indentthe substrate surface, the glass beads remain largely undamaged and thelayer between the beads and the substrate is not significantly damagedor compressed.

The effectiveness of the platen is quite unexpected because the damageto the glass beads and misalignment of the beads with the conformingconvex mirrors encountered in original attempts to form retro-reflectivesurfaces on hard substrates was believed to be due to the load requiredto force each bead to indent the substrate material via the interveningpolymer layer. If a platen is used, the beads must be subject to thesame loading in order to cause proper indentation, so it wasunreasonable to expect a platen to prevent damage to the beads.

The platen may take the form of any resilient or deformable materialhaving substantially no tendency to adhere to the glass beads during theindentation step. The material of the platen should be sufficiently softto be indented either elastically or plastically by the glass beadsduring the indenting step, but of course should be capable oftransmitting sufficient force to the beads to cause the necessaryidentation of the substrate.

Thus the platen may be a plate or sheet of aluminum or other metal. Ithas also unexpectedly been found that the platen can be a thin foil orweb of metal or paper or similar material. Such thin foils or webs areparticularly advantageous because they can be withdrawn from a roll ofthe material and passed virtually continuously through a roll mill withthe beaded substrate. This makes the manufacture of the retro-reflectivesubstrate economical particularly as the foil or web-like platens arethemselves inexpensive.

Although the use of a platen eliminates damage to the glass beads duringthe indentation step in the case of most metal substrates, somesubstrates (especially ferrous metals) are so hard that the loadingrequired to produce indentation cannot be transmitted by the beadswithout considerable damage, even when a platen is used. Thesesubstrates can easily be identified from hardness tables and from simpletrial and error. It is not possible to provide a maximum hardness limitbeyond which the method of the invention cannot be effectively operated,because the limit varies somewhat with the particular type of pressingequipment, glass beads, platens, operating speeds, etc. However, asstated, the useful metal substrates will be readily identifiable bypersons skilled in this technology. The term "indentable metalsubstrate" is used throughout this specification to refer to thosesubstrates which can be used in the method of the invention, i.e., thosehaving a hardness below the practical maximum hardness limit in theparticular operating conditions.

Very hard metals can be provided with a retro-reflective surface if thehard metals are first coated or "clad" with a softer metal because onlythe hardness of the surface layer of the substrate is important in theindentation step. The term "indentable metal substrate" thereforeincludes such structures.

It is also to be noted that the term "transparent" is used in a widesense throughout this specification and is intended to include materialsthat are sometimes referred to as semi-transparent. The important pointis that the various layers overlying the substrate, and even the glassbeads themselves, must be capable of transmitting sufficient light forthe structure to function effectively as a retro-reflector. Any layer ofmaterial (e.g., pigment or dye) capable of transmitting sufficient lightto achieve this function is considered to be "transparent" in thecontext of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below in which reference ismade to the preferred embodiments of the invention and to theaccompanying drawings, in which:

FIG. 1 is a diagram showing the apparatus used to measure theretro-reflective properties of various samples according to theinvention and control samples;

FIG. 2 is a graph showing the relative intensity of reflected lightversus the angle of incidence for samples according to the invention andcomparison samples;

FIG. 3 is a cross-sectional photomicrograph of a non-rolledretro-reflective surface layer;

FIG. 4 is a cross-sectional photomicrograph of the retro-reflectivesurface layer of FIG. 3 after rolling in accordance with the invention;

FIG. 5 is a cross-sectional photomicrograph of a prior artretro-reflective tape for comparison;

FIGS. 6 to 10 are graphs similar to FIG. 2 showing the reflectiveproperties of samples prepared according to the present invention;

FIG. 11 is a photomicrograph of the surface of a retro-reflective layeron an Alcan SW-30 alloy rolled with a platen;

FIG. 12 is a photomicrograph of the same retro-reflective layer of FIG.11 rolled without a platen;

FIG. 13 and FIG. 14 are photomicrographs similar to FIGS. 11 and 12,respectively, showing retro-reflective layers formed on AA5454 aluminumalloy;

FIGS. 15 and 16 are cross-sectional photomicrographs of retro-reflectivelayers formed on AA5454 aluminum alloy rolled with and without a platen,respectively;

FIGS. 17 and 18 are graphs similar to FIG. 2 showing the reflectiveproperties of various samples produced according to the invention usingvarious types of foil as the platen; and

FIG. 19 is a graph similar to FIG. 2 showing the retro-reflectiveproperties of the sample of Example 5 and two prior art tapes.

DETAILED DESCRIPTION

In the preferred method of the present invention, a metal substratehaving a retro-reflective surface can be prepared without carrying outthe disadvantageous steps of first forming an adhesive tape having theretro-reflective characteristics followed by adhering the tape to thesubstrate.

The indentable metal substrate is preferably first coated with atransparent layer of an organic polymeric material and the layer isallowed to harden or is cured. A second layer of an organic material isthen applied as an adhesive material. Small glass beads are applied tothe adhesive surface to form a mono-layer, the beads being as denselypacked on the surface as possible. The adhesive is allowed to harden oris cured. By proper choice of the transparent organic polymericmaterial, it is also possible to combine in one layer the functions ofspace coat and adhesive. In this case, the first layer is partiallydried or cured to a degree at which the glass beads applied to thesurface will adhere to form a mono-layer but will not sink excessivelyinto the polymeric material before it can be completely cured or dried.Thus the required separation between beads and substrate is preserved.

However formed, the structure is pressed in a mill so that the surfaceof the metal substrate is indented by the glass beads. Although theglass beads cause the metal substrate to form indentations, the beadsthemselves do not directly contact the substrate and remain spacedtherefrom by the polymer layer. The pressure applied to the beads duringthe indentation step is thus transmitted to the substrate layer via thepolymer layer and is sufficient to cause concave deformations to form inthe substrate metal.

By using a platen between the glass beads and the pressing roll or thelike, it has been found that suitable indentation of the surface can beobtained with little or no damage to the glass beads or to thetransparent layer separating the beads from the surface of the substratedespite the considerable pressure required to obtain the indentation.

It is not completely clear why the presence of a platen prevents damageto the retro-reflective structure but this may be because the platenacts as a cushion and, by spreading out the load, reduces the unitpressure on the glass bead surfaces. Alternately, or in addition, theplaten may avoid the relative motion between the beads and the pressureroll, or the like, which exists when no platen is used. Whichevermechanism is effective, the requirement for a platen was quiteunexpected and its effectiveness in preventing damage to the glass beadsand polymer layers is remarkable.

After the indentation step, the glass beads are covered with one or morelayers of a transparent organic polymeric material and cured or allowedto harden. The extra layer protects the glass beads and provides a flatouter surface. If the extra layer is omitted, a structure of the firsttype mentioned earlier would be produced which would lose itsretroreflective properties when wet with water.

Any one of the layers of organic polymeric material may contain apigment in order to impart a colour to the retro-reflective surface. Thepigment should preferably be light fast and not heat sensitive. Clearpigments give the best results, although any stable type of pigment canby employed provided the resulting pigmented layer remains transparentin the sense defined earlier in this specification.

The following general points should be considered when preparingretro-reflective substrates according to this invention:

(i) The refractive index and size of the glass beads affects the optimumdistance that the beads should be spaced from the substrate (i.e.,smaller beads and higher refractive indices require smaller optimumspacings from the substrate);

(ii) The rolling pressure applied to a given substrate metal affects thesize of the angle of incidence for effective retro-reflection (thegreater the pressure, the more the beads are indented into the substrateand the greater is the angle of effective retro-reflection); and

(iii) The layers of polymeric material applied over the glass beadsafter the indentation step should preferably be just sufficient toproduce a flat surface because thicker layers reduce the intensity ofthe reflected light.

The indentable metal substrates are usually metals suitable forpreparing objects such as road signs and markers and automobile licenceplates. Softer metals are preferred in order to reduce the pressuresrequired in the indentation step. Aluminum is the preferred material,although zinc-coated iron-base material has also proved very effective.The preferred aluminum alloys are soft alloys such as AA1100, althoughharder alloys like Alcan SW-30 (An Al-1/2% Mn type alloy) and AA5454(having a hardness of 85 BHN) can also be used. Since the angles ofincidence at which retro-reflectivity takes place may be reduced to someextent when harder substrates are employed because of reducedindentation of the metal, it may be desirable in those applicationswhere a very hard aluminum substrate is required to employ a cladsubstrate, i.e., a hard aluminum alloy as a core material and a softeralloy as a cladding.

Although, as stated above, there is an upper limit on the hardness ofthe substrates beyond which the method of the invention may not beeffective, there is no effective lower limit on the hardness of thesubstrates and very soft metals or metal foils can be employed ifdesired. The use of a platen is advantageous even in those cases inwhich the metal substrate may be soft enough to be indented at pressuresunlikely to produce significant bead damage, because bead misalignmentor displacement may still be caused if the substrates are indentedwithout the use of a platen.

The glass beads preferably have diameters in the range 50 to 80 micronsand are of high refractive index e.g., 1.9 and higher. Glass beadsobtainable from Flex-O-Lite Division of General Steel IndustriesLimited, St. Louis, U.S.A. under the designation Type 910-18 are foundto be useful. These beads have diameters of about 75 microns and arefractive index of 2.1.

Examples of other suitable glass beads are Type 831 (refractive index1.9), Type 938 (refractive index 2.28) and Fol Glass 1020 (refractiveindex 2.24) all available from Flex-O-Lite, and UGB 2.32 Toshiba fromWanami Abrasive Company Ltd., Tokyo, Japan.

As noted above, the lower the refractive index of the glass, the greateris the required thickness of the "space coat" (i.e., the first layer oforganic polymeric material). The size of the beads is also a factor indetermining the thickness of the space coat. For example:

(i) beads of 1.9 refractive index and screen size 200/270 (50-80μ)require a space coat 22-25μ in thickness;

(ii) beads of 2.1 refractive index and screen size 200/270 require aspace coat of 12-15μ in thickness;

(iii) beads of 2.24 and 2.28 refractive index and 230/270 screen sizerequire a space coat of 10-12μ in thickness;

(iv) beads of 2.32 refractive index and 230-325 screen size require aspace coat 8-10μ in thickness.

The required thickness of the space coat for each refractive index andbead size will be apparent to a person skilled in the art from thewell-known theoretical considerations.

Any suitable organic polymeric material having a strength when curedcapable of resisting the pressures encountered during the indentationstep without cracking or crazing can be used as the first layer oftransparent material separating the glass beads from the metalsubstrate. In particular, it has been found that thermosetting polymersare more likely to have the toughness to withstand the pressingoperation than thermoplastic polymers. An example is a thermosettingacrylic coating sold under the trade mark DURACRON 100 by CanadianPittsburgh Industries Ltd. The same material or a different material canbe used to form the other transparent layers of the product. Theimportant point is that the first layer (space coat) should be hard andnot susceptible to crazing, cracking or excessive attenuation during theindentation step, whereas the layers overlying the beads applied afterthe indentation step (top coat) need not be as hard but preferablyshould be able to release their solvent without "popping", i.e., thepolymeric material should preferably have a smooth surface when dry.

The transparent adhesive layer is usually quite soft so that it flowsupwardly around the beads to some extent during the indentation stepthus leaving the tops of the beads less proud from the surface thanbefore the application of pressure. Only a relatively thin top coat isthen required to be able to cover the tops of the beads and to form asmooth outer surface. The requirement for a thin top coat is preferredbecause thinner coats are less prone to "solvent popping" than thickercoats.

The adhesive layer should preferably be as thin as possible to reduceany tendency of the glass beads to form more than a single layer on theadhesive surface.

Additional examples of suitable organic polymeric coating materials arelisted as follows. While all of these materials are useful in theinvention, some have minor disadvantages as discussed below.

(i) Acryloid B-72 (Trade Mark) and Acryloid B-66 (Trade Mark)manufactured by Rohm & Haas. These thermoplastic materials are useful astop coats but do not meet the strength requirements of the first layer(space coat) subjected to the pressures of the indentation step.

(ii) Automotive lacquers. These materials have very low flash points andrequire air drying, which has the disadvantage of being slower than ovendrying.

(iii) DuPont 1234 (Trade Mark). This material also has a very low flashpoint and requires air drying.

(iv) Polyester and silicone polyester. These materials are particularlysuitable as the space coat and can also be used for the top coats. Asuitable solvent formulation can be provided to reduce their tendency to"solvent pop".

(v) U.C. cured resins. These materials are suitable as top coats buttend to be expensive.

(vi) Epoxy coatings. These materials are useful but generally have poorultraviolet resistance when used outdoors.

(vii) Water-borne acrylic emulsions such as the one sold by Rohm andHaas under the trade mark RHOPLEX E 1230. Such materials areparticularly advantageous because their use does not cause environmentalpollution.

(viii) Fluorocarbon coatings based on polyvinyl fluoride orpolyvinylidene fluoride resins are particularly suitable because oftheir characteristic good flexibility and durability.

(ix) As top coats, preformed polymer films can be used, eliminating theneed for special coating equipment and for drying and/or curing ovenswith their associated air pollution problems. These films can be appliedby standard laminating techniques employing adhesives coated on the filmor the glass beads. They can also be thermally bonded to the beads in a"hot nip" or by melting, without the use of adhesives. Additionally,some thermo plastic materials used as top coats can be applied directlyto the substrate in the molten state by standard extrusion coatingtechniques without the need to form a free film in a separate operation.

The indentation step can be carried out in a press mill or a rollermill, but a roller mill is preferred because this lends itself morereadily to continuous processes. Moreover, greater loads are requiredwhen the pressure is applied statically because the area of contact ismuch greater than in a rolling mill. The pressure required is dictatedto a large extent by the hardness of the substrate material, and theminimum pressure capable of producing satisfactory indentation ispreferably because higher pressures merely increase the possibility offracture of the beads or polymer layer. In the case of a mill having 4inch diameter rolls, the loading of the mill usually falls within therange of 140-800 kg/cm of substrate width.

As noted above, the platen may take the form of a sheet of metal, e.g.,aluminum, or may be a web of paper or a metal foil such as aluminumfoil. Any suitable type of paper or metal foil can be employed, althoughplastic coated papers should preferably be avoided because the plasticmay tend to adhere to the glass particles or press rolls. When paper ora foil is used as the platen, the process can readily be madecontinuous. For example, known coil coating techniques can be employedand the platen can be supplied to the press rollers from a large roll orthe like and can be rewound after passing through the press for disposalor re-use.

The surface of the indentable substrate should preferably be brightbefore the application of the space coat so that the metal surface formsa good reflector. However, the indentation step results in the formationof bright new metal surfaces within the concave indentations. It hasalso been found advantageous for the production of a retro-reflectivecoating with an improved white appearance, to "white-etch" the substratewith the well known appropriate chemicals, as a pretreatment to coating.

The invention will be described further with reference to the followingExamples which are not intended to limit the scope of the invention.

In these Examples, the retro-reflectance of samples was measured byusing the apparatus shown in FIG. 1. A light source 1, e.g., aphotoflood lamp or a spot lamp, shines through a circular aperturehaving a diameter of 5 cm onto a 10 cm square sample 3 ofretro-reflective material. The aperture serves to define the areailluminated and to block out stray light from behind the sample.

The sample is mounted on the axle of a synchronous motor 4 which causesthe sample to rotate at 1 rev/minute. Light reflected by the sample inthe direction of the light source 1 is caused to fall on a photocell 5.The light source and photocell are mounted 6 meters away from theaperture 2, and the photocell is protected fromm extraneous lightsources by a cardboard cylinder 6. The output of the cell is recorded ona suitable pen recorder 7.

To measure the intensity of retro-reflective light the sample is setrotating and the output of the photocell recorded. Zeroretro-reflectivity is taken as the light intensity when the sample isperpendicular to the aperture. The back of the sample holder is aspecular reflector which can be used to calibrate the chart speed of therecorder with sample angle. Since small changes in optical alignmentaffect the intensity measurement, a standard reflector sheet is measuredfor each set of samples.

Example 1

Two samples were prepared from clear (unpigmented) DURACRON 100 (TradeMark) thermosetting acrylic coating as the organic polymeric materialand Flexolite (Trade Mark) spherical glass beads of about 75 microns indiameter and having a refractive index of 2.1.

The samples were made by first applying (by drawdown) a base coat ofDURACRON 100 (Trade Mark) on panels of unpretreated mill finish AlcanSW-30 aluminum alloy sheet 0.56 mm thick, and curing it at 260° C. for 2minutes. A second very thin adhesive coating of DURACRON 100 (TradeMark) was then applied, allowed to partially dry for about 30 seconds at205° C., and then "beaded" by dropping on it a copious quantity of theFlexolite (Trade Mark) beads and shaking off those which did not adhere.This procedure produced a single densely populated layer of beadsadhering to, and partially embedded in, the surface. The adhesive coatholding the beads was then cured for 2 minutes at 260° C.

A panel of the same aluminum sheet of corresponding size was laid overthe beaded surface as a platen and the assembly passed through a sheetrolling mill for a nominal reduction of about 2 percent at a load ofabout 590 kg/cm of sample width. The platen was then removed and one ormore top coats of DURACRON 100 (Trade Mark) were applied and cured fortwo minutes at 260° C., thus filling in the interstices between thepartially imbedded beads and providing a transparent top coat over them.

The approximate dimensions of the various layers in the two samples aregiven in Table 1 below:

                  TABLE 1                                                         ______________________________________                                        Sample                                                                              Dry Film Thickness (Microns)                                            No.   Base Coat  Adhesive Coat                                                                              Top Coat*                                                                              Total                                  ______________________________________                                        186/3 20         4            29       104                                    189/2 20         4            42       117                                    ______________________________________                                         *The top coat thickness is the dry film thickness that would have been        applied with the drawdown bars used had there been no beads adhering to       the surface.                                                             

For comparison, identical samples (designated 186 and 189 respectively)were prepared except that the rolling (indentation) step was omitted.The retro-reflective properties of the rolled and non-rolledconstructions is shown in FIG. 2.

When the coating was removed from samples 186/3 and 189/2 it was readilyapparent that the aluminum substrate had been distorted into the form ofa concave mirror behind each bead. In confirmation of this,constructions 186 (non-rolled) and 186/3 (rolled) were sectioned andphotomicrographed with the results shown in FIGS. 3 and 4, respectively.For purposes of comparison, a similar photomicrograph of a prior artretro-reflecting adhesive tape is shown in FIG. 5.

Of particular interest is that the aluminum substrate has been distortedinto conformity with the beads without the base coat having been brokenor significantly thinned. That is, the spacing has been maintained whileenough conformity has been introduced to make the difference betweenhigh and low quality reflectors.

EXAMPLE 2

In this Example, retro-reflective layers were formed in a similar mannerto Example 1 on the following substrate metals as noted in Table 2 belowand were rolled with the platens and at the pressures also noted in theTable.

                  TABLE 2                                                         ______________________________________                                                                           Rolling                                                                       Mill                                                                   Beads  Load                                             Alloy, Temper         (Refrac-                                                                             (Kg/cm                                     Sample                                                                              and           Space   tive   of sample                                                                            Plat-                               No.   Thickness     Coat    Index) width) en*                                 ______________________________________                                        523   Alcan SW-30 H-15                                                                            8μ   2.28   500    Foil.                                     .023 in.                                                                550   Alcan SW-30 H-15                                                                            9μ   2.32   520    Foil.                                     .023 in.                                                                551   AA5454 H-36   8μ   2.32   700    Foil.                                     .051 in.                                                                546   AA5454 H-34   8μ   2.24   700    Foil.                                     .051 in.                                                                541/B AA5052 H-34   14μ  2.1    700    Foil.                                     .051 in.                                                                ______________________________________                                         *All foil platens were 0.0016 in. thick aluminum foil.                   

All samples formed acceptable retro-reflective layers with propertiessimilar to the best prior art tapes. A 40% higher rolling pressure wasrequired with the harder AA5052 and AA5454 aluminum alloys in order toindent the harder metal.

This Example shows that the harder alloys required for road signs andthe like can be made directly retro-reflective by the method of thepresent invention.

FIGS. 6 to 10 show the retro-reflective properties of these samplescompared with those of a commercial grade prior art tape. In the case ofsome of the alloys, higher intensity of reflection than the prior arttape can be achieved with some sacrifice of reflection at the largerangles.

EXAMPLE 3

In this Example, samples were prepared from Alcan SW-30 aluminum alloyand AA5454 H-36 aluminum alloy starting with a 5 in. ×12 in. ×0.051 in.sheet of the alloy material.

The following method steps were employed in the preparation of thesamples:

(a) A space coating of DURACRON 100 (Trade Mark, Canadian PittsburghIndustries), a thermosetting acrylic polymer, was applied to the sampleplate with a drawdown bar to a thickness of 8-10μ, and the coating wascured for 120 seconds at 260° C.;

(b) An adhesive coating of DURACRON 100 (Trade Mark) was applied overthe cured layer to a thickness of 5-8μ and was cured for 30 seconds at205° C.;

(c) Glass beads (refractive index 2.32, diameter 40-50μ, obtained fromWanami Abrasive Company, Ltd., Tokyo, Japan) were sprinkled on theadhesive coating while it was still hot;

(d) The bead coated sheet was cured for 90 seconds at 260° C.;

(e) Each sheet was trimmed to a width of 4 in., and, using a 0.0016 in.thick aluminum foil as a platen, each sheet was passed once through arolling mill having 4 in. diameter rolls, using a load of 700 kg/cm ofsheet width, and the platen removed;

(f) Each sheet then received two top coats of DURACRON 100 (Trade Mark)applied by a spray gun (producing a film which would have a thickness of25μ if applied to a flat sheet), the second coating being appliedimmediately after the first coat had been cured at 260° C. for twominutes.

The adhesive coating of step (b) was kept as thin as possible to avoidany "second layer" beads adhering to the samples.

To show the effectiveness of the use of a platen, the samples were eachrolled in the 4 in. mill with the platen arranged to cover only half thesurface of each sample.

Photomicrographs were made of both surface areas and also of the crosssections. In all cases, the area rolled without the platen showedfractured beads, while the panel area protected by the platen showed thebeads intact.

The photomicrographs of the samples prepared in this manner are shown inFIGS. 11 to 14, wherein

FIG. 11 shows the surface of an Alcan SW-30 alloy sheet coated withbeads and rolled with the protection of a platen;

FIG. 12 shows the same Alcan SW-30 alloy rolled without the protectionof a platen;

FIG. 13 shows an AA5454 alloy rolled with a platen; and

FIG. 14 shows the same alloy rolled without a platen.

All of the photomicrographs were taken at 160 times magnification.

Cross-sectional photomicrographs also show the benefit of using aplaten. These are shown in FIGS. 15 and 16, in which;

FIG. 15 shows the effect of rolling an AA5454 alloy with a platen;

FIG. 16 shows the effect of rolling the same AA5454 alloy without aplaten.

Both these photomicrographs were taken at 400 times magnification.

These photomicrographs speak for themselves. It is clear from them thatin all cases damage and dislocation of the glass beads results whenrolling is effected without a platen, but this can substantially beavoided when a platen is used.

EXAMPLE 4

In this Example, samples were prepared in a similar manner to Example 3.

The samples were rolled using platens of different weight and quality ofpaper and foil to determine their effect on the retro-reflectiveproperties.

The alloy used for the samples was Alcan SW-30 (H-15 temper, 0.023 in.thick). The thickness of the coating between the beads and the substratewas 8μ. The beads had a refractive index of 2.28 and the rolling millload applied was 500 kg/cm of sample width.

The reflective properties of the resulting samples are shown in FIGS. 17and 18, in which the dotted lines represent a commercial prior art tapeand the curve references indicate the foil platens shown below

K1--Freezer paper (0.004 inches)

K2--Kraft 10 lbs. (0.0025 inches)

K3--Wrapping paper (0.003 inches)

T--Tracing paper (100% rag, 0.0025 inches thick)

B--Bond paper (100% rag, 0.005 inches)

F1--Aluminum container foil (0.0056 inches thick)

F2--Aluminum converter foil (0.0016 inches thick)

It can be seen from the graphs that no significant difference can beattributed to the type or quality of platen. However, it is desirable tohave stronger paper or foil when using higher pressures.

EXAMPLE 5

A pretreated panel of Alcan SW-30 aluminum (size 4 inches ×10 inches×0.022 inches) was coated with 8μ of clear thermosetting acrylic lacquerand cured. A second coat of 8μ acrylic served as an adhesive for theglass beads having a refractive index of 2.28 (size 53 to 63) which weredusted on the adhesive layer to form a single, densely populated layer.The beaded panel was then cured.

Aluminum foil (0.0016 inch thick) of the same size as the panel wasplaced on top of the beaded surface and fed into a rolling mill loadedsufficiently to produce a 2% reduction in thickness at a load of about590 kg/cm of sample width.

The panel was removed and was coated with 25μ of clear acrylic lacquer.

The retro-reflective properties of the beaded coated sheet thus produced(designated sample 526) are shown in FIG. 19 and compared with twocommercially available retro-reflective tapes.

As can be seen from the graph, the product of this Example comparesfavourably with the better of the two commercially available tapes.

We claim:
 1. A method of forming a retro-reflective surface on anindentable metal substrate, comprising the steps of:(a) forming on saidindentable metal substrate a layer of transparent organic polymericmaterial having a mono-layer coating of substantially spherical glassbeads of high refractive index; (b) covering said beads with a platenhaving substantially no tendency to adhere to the glass beads under thepressures encountered; (c) applying sufficient pressure to said platenand indentable metal substrate to cause the glass beads to indent thesurface of said indentable metal substrate; and (d) covering said beadedlayer with a further layer of transparent organic polymericmaterial;said transparent organic polymeric material formed on theindentable metal substrate in step (a) being suitable to withstand thepressure of step (c) without substantial crazing, cracking orattenuation, and the thickness of said layer being sufficient to spacesaid beads from said substrate by a predetermined distance suitable forretro-reflection after step (c).
 2. A method according to claim 1wherein the platen is flexible metal plate or sheet.
 3. A methodaccording to claim 1 wherein the pressure is applied by passing thesubstrate and platen through a roller mill.
 4. A method according toclaim 3 wherein the mill applies a load within the range of 140-800kg/cm of the substrate width during the rolling operation.
 5. A methodaccording to claim 1 or claim 3 wherein the platen is a paper web.
 6. Amethod according to claim 1 or claim 3 wherein the platen is a metalfoil.
 7. A method according to claim 1, claim 2 or claim 3 whichcomprises employing a material selected from the following group as theindentable metal substrate material, said group consisting of aluminumalloys designated AA1100 and AA5454, an Al-1/2% Mn alloy, andzinc-coated, iron-base materials.
 8. A method according to claim 1,claim 2 or claim 3 wherein a thermosetting polymeric material isemployed as the layer applied to said substrate in step (a).
 9. A methodaccording to claim 1, claim 2 or claim 3 wherein the polymeric materialof step (a) is an acrylic resin which is applied to the substrate in theform of an aqueous emulsion.
 10. A method of preparing aretro-reflective surface on an indentable metal substrate, whichcomprises:(a) forming a layer of transparent organic polymeric materialon the indentable metal substrate; (b) applying a layer of transparentadhesive to the polymeric layer; (c) forming a mono-layer of sphericalglass beads of high refractive index on the adhesive layer; (d) coveringthe layer of beads with a platen; (e) applying sufficient pressure tothe platen to cause the glass beads to indent the surface of thesubstrate; and (f) covering the coating of beads with a further layer oftransparent polymeric material.
 11. In a method of preparing aretro-reflective surface on an indentable metal substrate, the stepsof:(a) disposing on said substrate a layer of transparent material, amono-layer of glass beads of high refractive index adhered to thetransparent material, and a platen over the layer of beads; (b) applyingsufficient pressure to the platen to cause the beads to indent thesurface of the substrate; and (c) removing the platen while leaving thebeads adhered to the transparent material and spaced from the substrateby a distance, of transparent material, suitable for retro-reflection.12. A method according to claim 11 in which the beads are adhered to theaforesaid transparent material by providing an intermediate layer oftransparent adhesive.
 13. A method according to claim 11 or claim 12which includes providing a further layer of transparent material overthe layer of beads.